Intent to ship: support content-box and stroke-box on transform-box CSS property

[Boris Chiou]

As of fx122, I intend to turn "transform-box: content-box | stroke-box" on by default on all platforms. It has been developed behind the layout.css.transform-box-content-stroke.enabled preference. Status in other browsers is:

1. Blink: Shipped, per wpt.fyi on stable,
   
2. WebKit: Shipped per the link above.
   

Bug to turn on by default: https://bugzilla.mozilla.org/show_bug.cgi?id=1868374.

Standard: https://www.w3.org/TR/css-transforms/#transform-box

The web-platform-tests: https://github.com/web-platform-tests/wpt/tree/master/css/css-transforms/transform-box

Basically, transform-box has been shipped by https://bugzilla.mozilla.org/show_bug.cgi?id=1208550 and we have an "intent-to-ship" thread in 2017 for it (but it is too old so I cannot get the link from google groups). This preference extends the property to support all values defined in the spec.

Regards,
Boris